1. Field of the Invention
The present invention relates to a fuel injection system for an internal combustion engine. More particularly, the present invention relates to a fuel injection system in which injection valves have been provided on an upstream side and on a downstream side, respectively, with a throttle valve interposed therebetween.
2. Description of Background Art
When a fuel injector is provided upstream from a throttle valve, the volumetric efficiency is improved because heat is taken from intake air when injection fuel vaporizes. Therefore, the engine output can be increased as compared with when the fuel injector is provided downstream from the throttle valve. On the other hand, when the fuel injector is provided on the upstream side, a distance between the fuel injection port and the combustion chamber inevitably becomes increases. Accordingly, a response lag occurs in fuel transport as compared with when the fuel injector is provided downstream from the throttle valve. This causes the driveability of the engine to deteriorate.
Japanese Patent Laid-Open Nos. 4-183949 and 10-196440 have attempted to solve such technical problems. Specifically, the above documents have attempted to improve engine output, while ensuring that the driveability of the engine and the engine output are compatible. The above documents disclose a fuel injection system in which fuel injectors have been provided on the upstream side and on the downstream side of the intake pipe, respectively, with the throttle valve interposed therebetween.
FIG. 7 is a cross-sectional view showing a major portion of an internal combustion engine according to the background art. Two fuel injectors 50a and 50b have been arranged with a throttle valve 52 of an intake pipe 51 interposed therebetween. The downstream fuel injector 50a is arranged on a side portion of the downstream side (engine side) of the throttle valve 52 and the upstream fuel injector 50b is arranged on the upstream side (air cleaner side) of the throttle valve 52. A lower end portion of the intake pipe 51 is connected to an intake passage 52. An intake port 53, which faces a combustion chamber of the intake passage 52, is opened and closed by an intake valve 54.
In Japanese Patent Laid-Open No. 8-135506, a technique has been disclosed in which a hot water passage has been formed on the throttle body in the vicinity of the intake passage. The hot water passage is provided for circulating engine cooling water, and the cooling water heated by the engine is caused to circulate in the hot water passage to thereby heat the throttle body for preventing the throttle body from freezing.
In the above-described technique; however, piping is required for introducing the engine cooling water to the throttle body to circulate from the engine body through the throttle body. Such piping requires a complicated structure to conduct a large quantity of heat from the engine body to the throttle body. Therefore, the space required for the installation of the throttle body and the weight increases, and the assembly process becomes complicated. In view of this, the manufacturing costs increase.
It is an object of the present invention to solve the above-described problems of the background art. Specifically, it is an object of the present invention to provide, a fuel injection system for an internal combustion engine which is capable of preventing the throttle valve from freezing without involving the addition of piping and the like in a structure in which fuel injectors are arranged on the upstream side and on the downstream side of the throttle valve, respectively.
In order to achieve the above-described object, a fuel injection system for an internal combustion engine according to the present invention includes an upstream fuel injector provided upstream from the throttle valve and a downstream fuel injector provided downstream from the throttle valve. The fuel injection system comprises means for determining a total injection quantity due to the upstream and downstream fuel injectors; means for determining a rate of fuel injection quantities due to the upstream and downstream fuel injectors; means for acquiring temperature information representing temperature of the throttle valve; and means for correcting the rate on the basis of the temperature information, wherein the correction means decreases the injection rate of the upstream fuel injector when the temperature of the throttle valve is lower than a predetermined temperature.
According to the above-described feature, when the throttle valve is at low temperature, the injection rate of the upstream fuel injector is restricted to a low amount. Accordingly, the quantity of fuel to be injected to the throttle valve is reduced. As a result, the total quantity of the heat of vaporization to be taken when the fuel vaporizes is restricted to a low value. Accordingly, the throttle valve can be prevented from freezing. In addition, the total injection quantity due to the upstream and downstream fuel injectors is maintained constant. In view of this, it is possible to prevent fuel shortages due to the injection quantity of the upstream fuel injector being reduced.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.